1. Field of the Invention
The present invention pertains to a copolymer containing a first repeat unit derived from a fluoroolefin and a second repeat unit derived from an ethylenically unsaturated monomer containing an acid group or a protected acid group which cocopolymer is useful in a photoresist composition for imaging in the production of semiconductor devices.
2. Background
In the process for manufacturing semiconductor devices, very fine features are etched onto a substrate, typically a silicon wafer. The features are formed on the substrate by electromagnetic radiation which is impinged, imagewise, on a photoresist composition applied to the silicon wafer. Areas of the photoresist composition which are exposed to the electromagnetic radiation change chemically and/or physically to form a latent image which can be processed into an image for semiconductor device fabrication. Positive working photoresist compositions generally are utilized for semiconductor device manufacture.
The photoresist composition typically is applied to the silicon wafer by spin coating. The silicon wafer may have various different material layers applied to it in other processing steps. In addition, in a particular photolithographic processing step, the silicon wafer may have a hard mask layer, typically of silicon dioxide or silicon nitride, applied below the photoresist composition layer. In addition, an antireflective layer (ARC) may be applied below the photoresist composition layer, by a coating process (and is then typically referred to as a bottom anti-reflective (BARC)) or on top of the photoresist composition layer (and typically called a top-anti reflective layer (TARC)). Typically the thickness of the resist layer is sufficient to resist the dry chemical etch processes used in transferring a pattern to the silicon wafer.
Various copolymer products for photoresist compositions have been described in Introduction to Microlithography Second Edition by L. F. Thompson, C. G. Willson, and M. J. Bowden, American Chemical Society, Washington, DC, 1994.
The photoresist composition generally comprises a film forming copolymer which may be photoactive and a photosensitive composition that contains one or more photoactive components. As described in the Thompson et al. publication, upon exposure to electromagnetic radiation (e.g., UV light), the photoactive component acts to change the rheological state, solubility, surface characteristics, refractive index, color, electromagnetic characteristics or other such physical or chemical characteristics of the photoresist composition.
The use of ultraviolet light of lower wavelength corresponds to higher resolution (lower resolution limit). Lithography in the UV at 365 nm (I-line) is a currently established image-forming process for making semiconductor devices. The features formed by this process have a resolution limit of about 0.35-0.30 micron. Known photoresist compositions for lithography using a 365 nm wavelength are made from novolak copolymers and diazonaphthoquinones as dissolution inhibitors. Lithography in the deep UV at 248 nm has been found to have a resolution limit of approximately 0.35-0.13 micron. The known photoresist compositions for this process are made from p-hydroxystyrene copolymers. Lithographic processes using electromagnetic radiation at even shorter wavelengths are looked to for forming very fine features because the use of lower wavelengths correspond to higher resolution; that is, in deep (wavelength less than 300 nm), vacuum (wavelength less than 200 nm) or even the extreme (wavelength less than 30 nm) ultraviolet. However, at wavelengths of 193 nm or shorter, the photoresist compositions known for use at 365 nm and 248 nm have been found to lack sufficient transparency.
A key difficulty encountered in developing copolymers for use in photoresist compositions that are imaged at lower wavelengths, e.g., 157 nm, is the lack of transparency at these low wavelengths. The transparency requirements for photoresist compositions are usually on the order of allowing about 20 to about 40% of incident light to penetrate the full thickness of the resist layer to produce an image with well-defined, vertical side walls, which are important in achieving high resolution and minimizing defects. Copolymers which lack transparency absorb too much light and thereby produce an unacceptable image with low resolution and too many defects.
A thick film layer of the photoresist material is beneficial for high resolution and low defects. However, thick films tend to lack transparency at low wavelengths. To determine the absorbance requirements of a copolymeric material that will allow about 20 to about 40% of incident light to penetrate the resist film layer, the light transmission (T) for a thin copolymeric resist film layer, considering the effects of optical interference in the film, was calculated as a function of the film thickness in angstroms (t) and the optical absorbance per micron (of film thickness) of a film layer containing the copolymer. The results are shown for a copolymeric resist for use at 157 nm in FIG. 1 and for a copolymeric resist for use at 193 nm in FIG. 2. In this calculation an index of refraction (n) for the resist of 1.6 is used and no effect(s) of the underlying silicon substrate, representing the light transmitted through an unsupported copolymer film layer have been considered to represent the light transmitted through an unsupported film. As shown in FIG. 1, that for films of approximately 1300 angstroms thickness to achieve a transmitted light intensity at 157 nm of about 20% can require a copolymer film with an absorbance per micron (of film thickness) at 157 nm below 5 per micron (<5 μm−1). For use at 193 nm, as shown in FIG. 2, that for films of approximately 5500 angstroms thickness, to achieve a transmitted light intensity at 193 nm of about 25% can require a copolymer film with an absorbance per micron (of film thickness) at 193 nm below 1 per micron (less than 1 μm−1), while for a film thickness of 4000 angstroms, a copolymer film having an absorbance per micron of 2 per micron (2 μm−1) can have a transmitted light intensity at 193 nm of only about 14%. Accordingly, the thicker a film gets, the lower absorption it must have to transmit a reasonable percentage of incident short wavelenght light.
The molecular weight of a polymer for a photoresist composition which is sufficient for high film-forming properties may be detrimental to transparency at low wavelengths. Table 1 below shows the known absorption maxima for simple hydrocarbon H(CH2)nH and fluorocarbon F(CF2)nF chains. As these chains become longer, their absorption moves to longer wavelengths, first starting up at 157 nm for hydrocarbons when n is greater than 1 and for fluorocarbons when n is greater than about 10. This implies that polymers having (CH2)n chain segments with n greater than 1 and (CF2)n segments with n greater than about 10 are likely to be highly absorbing at 157 nm. One of the problems faced therefore by one wanting to make resists highly transparent to 157 nm light is how to achieve polymeric molecular weights, especially molecular weights sufficient for film-forming capability, while avoiding long (CH2)n and (CF2)n chains which hinder tranparency.
TABLE 1Comparison of UV Absorption Maxima forHydrocarbons and FluorocarbonsWAVELENGTH OF ABSORPTION MAXIMUMX(CX2)nXX = H1X = Fn = 1143 nm & 128 nmn = 2158 nm & 132 nmn = 3159 nm & 140 nm119 nm1n = 4160 nm & 141 nm126 nm1n = 5161 nm & 142 nm135 nm1n = 6162 nm & 143 nm142 nm1n = 7163 nm & 143 nmn = 8163 nm & 142 nmN = 172161 nm21B. A. Lombos, P. Sauvageau, and C. Sandorfy, Chem, Phys. Lett., 1967, 42. 22. K. Seki, H. Tanaka, T. Ohta, Y. Aoki, A, Imamura, H. Fujimoto, H. Yamamoto, H. Inokuchi, Phys. Scripta, 41, 167(1990). 
Even when a copolymer film for use in imaging at 157 nm is only 1500 Å thick, very few organic copolymers have been found that allow about 20 to about 40% of 157 nm light to penetrate or, that is, that have an (optical) absorbance per micron (absorbance (A) per micron of film thickness t) of less than 3 μm−1. For example, of 12 candidate copolymer systems tested, Bloomstein et. al. (J. Vacuum Sci. Technol., B16, 3154 (1998)) report only three copolymers having an absorbance per micron less than 3 μm−1. These three copolymers are Teflon™ AF having an absorbance per micron of about 0.5 μm−1, poly(methysiloxane) having an absorbance per micron of about 1.5 μm−1, and poly(phenylsiloxane) having an absorbance per micron of about 2.5 μm−1. Attempts to convert these pure copolymers into thin films both sensitive to light and capable of aqueous base development have tended to increase absorption at 157 nm. Indeed, none of the three transparent copolymer systems found by Bloomstein have yet been successfully converted into practical resists. There is a need for photosensitive, aqueous base developable copolymers sufficiently transparent to UV light to make a good resist. Resists effective at 157 nm should also work at longer wavelengths (for example, 193 and 248 nm).
Because photolithography at the shorter wavelengths would provide the very fine features having lower resolution limits; that is, a resolution limit of approximately 0.18-0.12 micron at 193 nm, approximately 0.07 micron at 157 nm photoresist compositions that will be sufficiently transparent at these short wavelenths are needed.
There is a need for suitable photoresist compositions for use at 193 nm and lower, particularly at 157 nm, that have not only high transparency at these short wavelengths but also other key properties, including good plasma etch resistance, development characteristics, and adhesive properties.